DEVELOPMENT OF THE ALBINO RAT 25k 
of observed coitus. The slight though observable variation in 
the rate of development in a series of ova of the same animal, 
more marked when supposedly similar stages of several animals 
are investigated, precludes the accurate timing of stages. 
As fixing fluids, there were used Zenker’s fluid, sublimate- 
alcohol, Flemming’s fluid, Bouin’s fluid, and Carnoy’s fluid. 
After a few trials, all were discarded in favor of Carnoy’s fluid, 
prepared by mixing 6 parts of absolute alcohol, 3 parts of chloro- 
form, and 1 part of glacial acetic acid. This somewhat illogi- 
cally compounded fluid penetrates rapidly and does not cause 
shrinkage. ‘Tissues are fixed in it for several hours, then washed 
in several changes of absolute alcohol in which it has been my 
custom to store the tissues. The following procedure was prac- 
ticed in all stages up to about 12 days after insemination: The 
animals were anaesthetized and the head severed from the body, 
to admit of free bleeding. The rat was then fastened to a 
board, and thorax and abdomen opened by a mid-sagittal in- 
cision, the abdominal walls pinned back, and the intestine ele- 
vated toward the thorax. With as little manipulation as pos- 
sible, the ovaries were separated from their attachment, the 
mesometrium cut, the uterine horns elevated and the vagina 
severed. The whole genital tract was then placed on a clean 
slide and arranged in approximately normal position. Slight 
tension was maintained by tying a thread to the connective 
tissue removed with each ovary and bringing the threads along 
the reverse side of the slide and tying them to the vagina. If 
the slide is clean, the mesometrium of each uterine horn may be 
spread out evenly and caused to adhere to the slide. Ovaries, 
oviducts, and uterine horns may thus be spread out in normal 
position and each uterine horn fixed as a straight tube. When 
thus arranged on the slide, the preparation was placed in a 
relatively large quantity of Carnoy’s fluid, fixed, and then trans- 
ferred through several absolute alcohols. For nearly all the 
material used in this study, the method of fixation was as here 
given. In the earlier stages of material collection, attempts 
were made to obtain segmentation stages in warm normal salt 
solution. Several were thus obtained and were used to control 
